1. Field of the Invention
The invention relates to the field of data transfer to and from magnetic tape and in particular, relates to circuitry and methodology for reading data from magnetic tape independently of the data pattern recorded upon the tape.
2. Description of the Prior Art
Modern streaming tape drives are driven as high as 90 inches per second with recording densitities of approximately 10,000 flux reversals per inch, resulting in a data transfer rate in the group of binary bytes of approximately 90,000 bytes per second. Each bit is therefore recorded on less than 100 micro inches of tape (2.5 microns).
Binary information is recorded on magnetic tape by forming a north or south magnetic pole pattern in the thin ferromagnetic layer on the tape to indicate the presence of a signal or a binary one, and by leaving the magnetic domains unorganized to indicate the absence of a signal by the absence of a magnetic pole region to indicate a binary zero. Therefore, a blank tape is equivalent to a tape with prerecorded zeros at each data location. A data pattern recorded on the tape which includes data fields of adjacent binary zeros is then represented by a proportionate length of tape upon which no signals have been recorded. When dealing with data fields as small as those considered here, small variations in tape speed over the read-head can cause prior art phase-locked-loop circuitry to lose track of which data field is being read, thereby losing sequential order of data recorded on the magnetic tape. Such instantaneous speed variations occur due to the mechanical frequencies of resonances, characteristic of the tape head, tape drive, or tape cartridge or cassette. In addition, speed variations at higher frequencies arise from peak shifts on the magnetic tape which inherently result when small magnetized domains are densely packed on the magnetic tape adjacent to blank or nonmagnetized domains.
Typical prior art phase-locked-loop tape readers adjust a read window to compensate for such speed variations of the data field across the magnetic head by matching the read data in a phase comparator with an output frequency from a voltage controlled oscillator. Any time displacement error between the read pulse and a reference edge from the voltage controlled oscillator is amplified and fedback to the voltage controlled oscillator to adjust the output of the oscillator to minimize TDE. Thus, the output of the voltage controlled oscillator is constantly adjusted in an attempt to match the transfer rate of the read data. This adjusted output, proportional to measured TDE, is then used in conventional circuitry to establish a time window within which to detect the presence or absence of the pulse on the magnetic tape. The detector generates an output which is then used to generate a corresponding string of binary ones and zeros according to whether or not a data signal is detected during the defined time on the tape. As long as the adjusted time windows defined by the read circuitry correspond to the actual time windows of the data fields as read by the magnetic head, taking into account perturbations in measured speed of the tape across the head due to instantaneous speed variations data is read from the tape by the circuitry in a correct manner.
However, the voltage, as seen by the amplifier driving the voltage controlled oscillator, is a time average of the error signal generated by the phase comparator. Clearly, errors can be detected between the reference voltage and a signal recorded on the tape only where there is in fact a signal on the tape. As stated above, binary zeros are recorded on the tape as an absence of a signal. Therefore, no TDE is generated corresponding to a binary zero. A binary zero thus is not read in a sense that a signal is detected but by the fact that a signal is not detected during an assumed data window. Therefore, if an instantaneous speed variation error occurred during a string of binary zeros read from the tape, that error cannot be detected until a binary one is read.
Since a time average is taken to produce a feedback signal to the phase comparator, a fewer number of error signals would be found per unit time in the case where a data pattern consisted of a long string of zeros, than in the case where the data pattern was composed entirely of binary ones. Therefore, such prior art phase-locked-loop circuits produce a feedback signal which is dependent on the data pattern. This dependence on data pattern is seen as a dependence of the gain of the phase-locked-loop circuit upon the data pattern. It is desirable to be able to follow or compensate for the instantaneous speed variation errors and to ignore peak shift errors. Therefore, the phase-locked-loop circuit must have a high gain at those lower frequencies characteristic of instantaneous speed variations. It should have a bandwidth below the lowest frequency at which peak shift errors occur when the tape is run at its designed rate, typically 90 inches per second. However, when the gain of the phase-locked-loop circuit varies dependent on the data pattern, which is largely arbitrary within any data coding scheme, the characteristics of the phase-locked-loop gain can change enough so that the bandwidth falls below some instantaneous speed variation frequencies. The prior art has attempted to solve this problem by enhancing the gain of the phase-locked-loop circuit enough such that the largest decrease which would be caused by the data pattern, would still leave the bandwidth well above the highest instantaneous speed variation frequency. However, when such attempts have been made, often the bandwidth will be high enough such that peak-shift perturbations can then be observed and affect the read operation.
Therefore, what is needed is a circuit design and method for reading data from magnetic tape at high speeds and bit densities such that tracking of the fields on the magnetic tape can be accomplished without dependence on the data pattern.